Thermionic cathode structure



May 21, 1940. T. A. ELDER 2,201,721

THERMIONIC CATHODE STRUCTURE Filed July 11, 1959 Inventor-z Thomea A.Elder;

is Attorney Patented May 21, 1940 UNITED STATES 2,201.7 21 THERLHONICCATHODE STRUCTURE Thomas A. Elder, Scotia, N. Y., assignor to GeneralElectric Company, a corporation of New York Application July 11, 1939,Serial No. 283,803

8 Claims.

The present invention comprises improved cathodes for electricaldischarge devices, and in particular for heavy duty power devices suchas rectifiers and thyratrons for use in the industrial field. It is anobject of my invention to provide thermionic cathodes of ruggedconstruction and large emitting surface which are capable of beingheated quickly and efficiently to electron-emitting temperature.

My invention is applicable in particular to electrode structures inwhich elongated cathode members, such as ribbons, are embodied which aremaintained at electron-emitting temperature by radiant heat. Suchstructures should be mechanically rugged and supported so firmly thatdisarrangement of parts which might establish contact between the heaterand the emitting surfaces is precluded. In power devices particularly,thermionic cathodes should provide for an open path to the anode for theelectron discharge from all parts of the emitting surfaces of thecathodes. A generally radial arrangement of such surfaces as hereinshown provides an open path.

It is particularly desirable that the heavy duty cathodes in power tubesshould be substantially unipotential in order that the load current willbe equally distributed over the emitting area. It is further desirablethat the electron-emitting members should be maintained at operatingtemperature with the least expenditure of heating energy. As will beshown, these and other desirable features are present in cathodesembodying my invention.

As will be pointed out with greater particularity in the appendedclaims, cathode constructions embodying my invention are characterizedby the combination of a cathode framework providing a plurality ofsupports located in different planes upon the periphery of which one ormore elongated members are mounted and constitute a unipotentialelectrode of open structure having generally planar emitting surfaces ofsubstantial area. A cathode heater is supported within such cathodewhich is surrounded by a heat-conserving enclosure.

An embodiment of my invention is illustrated by the accompanying drawingin which Fig. 1 is a side elevation of a rectifier in which an improvedcathode assembly is shown, partly in section; Fig. 2 is a perspectiveview on an enlarged scale of an electron-emitting cathode structureembodying my invention; and Fig. 3 illustrates a constructional detail.

Referring to Fig. 2, the cathode framework comprises two rings 2, 3,which are held in spaced relation by electrically conducting rods 4, 5,6 and I. The rings 2, 3 which may consist of metal, such as nickel forexample, each are provided with a plurality of radially projectingsupports or pins, which may consist of a more refractory metal, such,for example. as molybdenum.

The pins, carried by the upper ring 2-, which are designated by thenumeral 8-, and the pins, carried by the lower ring 3, which aredesignated 10 by the numeral 9, are arranged in staggered relation toone another as shown. A crimped ribbon I0 is looped back and forth overthese pins and forms a series of V-shaped loops arranged in spacedzigzag relation to form a cage-shaped structure with relatively widespacing between the ribbon elements. As will be observed from thedrawing, corresponding edges of the electron emitting cathode elements,that is, the adjacent inner edges and the adjacent outer edges re--spectively, of the ribbon ID are spaced apart from one another, leavingrelatively wide openings therebetween. Preferably the cathode ribbon isfastened to the two sets of pins 8 and 9 by spot welding, or otherwise,although in some cases frictional contact will be sufiicient to hold theribbon in place. The ribbon in, which may consist of nickel, molybdenum,or other suitable metal, preferably is coated or otherwise activated toincrease electron emissivity. Ordinarily it is coated with alkalineearth oxide, such as barium,

strontium or calcium oxide, or a mixture of such length of cathoderibbon, it is immaterial whether the ribbon consists of a single lengthdraped back and forth over the pins and fastened thereon, or whether itconsists of a plurality of sections which are electrically connected tothe conduct- 4 ing framework. In either case a unipotential annularstructure, mechanically and electrically connected at numerous points,will result. In such a structure the discharge current does notconcentrate at a region near a terminal, which would result in shortenedlife.

Although a crimped ribbon is preferred for the purposes of my invention,other generally planar forms of emitting structures not only uncrimpedbut otherwise differing from the form shown may be used. A plurality offilaments arranged side by side could be used to constitute a desiredplanar structure. The emitting structure may be twisted or otherwisemodified in shape. As indicated in the drawing, wire gauze may be usedas the base member which is coated with activating material. likewise,gauze may be applied to the surface of a solid ribbon to assist inmaintaining coating material in place and to increase the surface, or aplain metal ribbon or a perforated metal ribbon may be used.

The ribbon cathode is heated by radiation to an electron-emittingtemperature, for example, to about 850 to 950 C. For this purpose ahelical filamentary heater l2, Figs. 1 and 3, is provided in the spaceenclosed by the cathode. This heater, which may consist of tungsten, orother suitable refractory material, may be supported at one end by atransverse bar I3 (Fig. 3) which is joined by welding or otherwise tothe supports 4 and 6, or is supported otherwise by the cathodeframework.

As shown in Fig. 1, the cathode ribbon is surrounded by a metal heatshield i4, shown partly broken away and in section. It preferably is astructure providing spaced walls i5, i8, between which are located aplurality of layers of metal foil I! held in loosely spaced relation asdescribed in U. S. patent application Serial No. 243,101 filed November30, 1938. The heat shield is described in British Patent No. 383,645 ofNovember 24, 1932. The effective heat insulation of such a structure isgreater than a single-walled shield,

or a shield with a lesser number of walls. As a consequence, the insidewall of the shield is so highly heated that the cathode receives heateffectively by radiation from the highly heated inner shield wall aswell as from the primary heater II. The shield also includes anapertured cover I8.

Fig. 1 shows the cathode and appurtenant parts mounted in a bulb l9.There is provided as usual also an anode 20 which is connected by a lead2| to an external contact 22.

The assembly comprising the cathode and the heat shield is mounted on aglass stem 24 by the sealed-in conductors 25, 26 and 21, and the shieldsupport 28. The conductors 25, 26 and 21 are connected to the usualexternal contact plugs or prongs 29. The base 30 is of conventionalstructure.

In a rectifier only three external contacts are required, the fourthmerely serving to properly orient the plugs in the socket. Should thedevice contain also a grid (not shown), then the grid conductor isconnected to one of the contact plugs. Two conductors, for example, 26,21, conduct energizing current to the heater filament.

. These conductors are connected to the external circuit 3| whichcontains a suitable source of current such as the secondary of atransformer 32, the primary of which is supplied by the conductors 33.The third conductor 25 leads to the cathode only and serves to carryload current. It is connected to the external conductor 34. The loadconductors 34 and 35 are connected to a transformer 36, the load beingrepresented by a resistance 31.

The bulb I9 is freed from water vapor and is evacuated in the usualmanner through the exhaust opening 39, a getter in a capsule 40 beingprovided to remove residual gas. The bulb is charged before sealing witha low pressure gas or vapor. For example, a small globule of mercury maybe provided as indicated at 4|, the pressure of mercury vapor at anoperating bulb temperature of 40 to C. being about 6 to 24 microns.However, the described construction is not limited to a specificpressure of ionizable medium.

The shield it does not take part in the emission of electrons.Insulators 42, 43 are provided at the bottom and the heater conductor 28passes through a clearance hole in the bottom of the inner shield wallas indicated. The shield it may be connected to the cathode by a circuit44 containing a high resistance 46 (for example, 5

10,000 ohms or more), or this circuit and the resistor may be omitted,the shield being electricglly insulated from the electron-emitting cathoe. I

My invention, however, does not require as an essential feature anyparticular form of heat shield structure.

While my improved cathode has been illustrated in connection with arectifier, that is, a two-element device, I wish it to be understoodthat it can be used in discharge devices having additional elements.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A thermionic cathode for electric discharge'zo devices comprisingsupports spaced apart, a continuous ribbon providing a plurality ofzigzag activated metallic ribbon elements mounted on the periphery ofsaid supports to form a cage-shaped structure, said ribbon elementshaving generally 25 planar emitting surfaces and being arranged spacedapart with openings between adjacent edges, and means for heating saidribbon elements to an electron-emitting temperature.

2. A thermionic cathode comprising a linearly 0 extending heater, amounting framework having two sets of radially extending supportingmeans arranged in axially displaced planes adjacent opposite ends ofsaid heater, a substantially continuous metallic structure mountedaround said heater in a zigzag course over and between said supportingmeans, said metallic structure being of such character as to define byits various zigzag sections a plurality of radially extending andgenerally planar emitting surfaces of substantial area.

3. In an electrical discharge device an assembly comprising an elongatedribbon-shaped elecvtron-emitting cathode, an electrically conductingframework therefor providing a plurality of outwardly extending supportsspaced apart in different planes, said cathode being mounted in a zigzagarrangement upon said supports thus forming a unipotential structure, anelectrically insulated heat-conserving enclosure therefor andheat-radiating means for heating said cathode to 0 an electron-emittingtemperature.

4. In an electrical discharge device a assembly comprising thecombination of a framework consisting of conductive support membersspaced apart, longitudinal rods mechanically and electrically connectingsaid members to one another, a plurality of pins projecting outwardlyfrom said members, an elongated planar electron emitter supported bysaid pins as a zigzag structure spaced about said rods, means forheating said electron emitter by radiant heat, and an electricallyinsulated, aperture heat shield surrounding said electron emitter.

5. In an electrical discharge device an assembly comprising aspool-shaped support including a pair of annular metal rings connectedtogether by metal tie rods extending at right angles to the planes ofsaid rings, said rings each being provided with a plurality of outwardlyprojecting pins, a metal ribbon wound in a zigzag course over said pins,said ribbon being welded to said pins at the regions of contact, afilamentary heater, means for supporting said heater in an axialposition with respect to said ribbon and 7;

a heat-conserving enclosure closely surrounding said ribbon.

6. A structural asembly for an electrical discharge device comprising anapertured heat shield, a filamentary radiation heater supported therein,substantially in line with the longitudinal axis of said shield, aframework within said shield providing a plurality of sets of supportingrings surrounding said heater, a set of outwardly projecting pins oneach of said rings, said sets of pins being spaced in staggered relationto one another, an electron emitter comprising an elongated meandrouscrimped ribbon supported by and electrically joined to said pins,thereby forming a substantially unipotential structure, a coating ofmaterial on said ribbon capable of increasing electron emission, anenergizing circuit for said heater and a load circuit cathode terminalwhich is independent of said heater circuit.

'7. A thermionic cathode comprising the combination of a supportingstructure including a pair of metal rings, elongate metal rodsconnecting said rings, and pins outwardly projecting from said rings, anelongate filamentary heater,

means for supporting said heater axially within said structure, andelectron-emitting ribbon elements supported on said pins in unobstructedradiation-receiving relation to said heater, said ribbon elementsextending between said rings in spaced relation with respect to oneanother, the sides of said ribbon elements being substantially radiallyarranged with respect to heater.

8. A structural assembly for an electrical discharge device comprisingthe combination of two sets of radially projecting supports spaced apartin dilferent planes, an electron-emitting ribbon mounted in a series ofloops on said supports, an elongated radiant heater extending betweensaid supports in the space enclosed by said loops, and an enclosing heatshield, said ribbon having a generally planar surface and being arrangedsubstantially radially about said heater with corresponding edges ofcontiguous sections spaced apart from one another leaving openingswhereby said ribbon is in direct receiving relation for heat radiatedfrom said heater and reradiated by said heat shield.

THOMAS A. ELDER.

